Certain known variable voltage gain amplifiers include operational amplifiers of both single stage and multi-stage designs. These conventional circuits often suffer from such performance limitations as limited dynamic range, distortion in the presence of heavy output loads, and tradeoffs between the level of voltage gain available and the bandwidth over which the amplifier provides a substantially flat voltage gain profile.
Referring now to FIG. 1a there is shown a schematic of a conventional single-stage variable gain amplifier 100. The amplifier includes a pair of n-type, field effect transistors (NFETs) 3, 4 connected to receive a pair of differential input signals for generating a pair of differential output signals 7,10 as amplified versions of the input signals. Each output signal 7,10 is produced across a p-type, field effect transistor (PFET) 5, 6 that provides an effective variable resistance in response to applied gain-controlling signal 9, for controlling the voltage gain provided by the amplifier.
In such conventional amplifiers, the level of voltage gain provided by the amplifier is substantially inversely proportional to the bandwidth of the amplifier. Further, if the amplifier is used to drive a heavy load at high gain, the amplifier may distort the output signals. The dynamic range provided by the amplifier is limited by the voltage differential across the drain and source of each respective PFET 5, 6. More particularly, the limit on output voltage swings is substantially limited to I.sub.PFET .times.R.sub.eff for each output voltage signal 7,10, where I.sub.PFET is the current from source to drain in each PFET 5, 6 and is dependent on the voltage level of the supply voltage 8 and the current level of the current source 17.
Referring now to FIG. 1b, there is shown a conventional multi-stage variable gain amplifier 101 including a plurality of cascaded single-stage amplifiers 20, 21, 22, 23 of a type such as shown in FIG. 1a, for amplifying applied differential input voltage signals 18, 19 in successive stages. Amplification of voltage signals in successively cascaded stages typically improves the bandwidth of the amplifier at various levels of voltage gain. However, because each of the single-stage, variable-gain amplifiers 21-23 is coupled to the same voltage gain control line 25, the voltage gain provided by the output stage amplifier 23 is increased as the voltage gain of the entire multi-stage amplifier 101 is increased, thereby reducing the ability of the output stage amplifier 23 to drive heavy output loads at high voltage gain and wide bandwidth.